2. The Process That Feeds the Biosphere
Photosynthesis:
- converts solar energy into chemical energy
- directly or indirectly feeds entire living world
- in plants, algae & other protists, some prokaryotes
3. Autotrophs: survive without eating
anything derived from other organisms
- Producers: make organic molecules
from inorganic molecules (H2O and CO2)
- Photoautotrophs: use solar energy to
make organic molecules
5. Heterotrophs: obtain their organic material
from other organisms
- Consumers of the biosphere
- heterotrophs depend on photoautotrophs
for food and O2
Can an organism be both autotrophic and
heterotrophic?
6. Chloroplasts are structurally similar to and likely
evolved from photosynthetic bacteria
- their structure enables the chemical reactions of
photosynthesis to occur – how so?
7. Leaves: the major organs of photosynthesis
- chlorophyll: green pigment inside chloroplasts
- absorbs light energy that powers the synthesis of
organic molecules
CO2 enters and O2 exits the leaf through stomata
8.
9. Chloroplasts: found in mesophyll cells in the leaf’s
interior
- 30-40 chloroplasts per mesophyll cell
Chlorophyll is in the thylakoid membranes of
chloroplasts
- thylakoids are stacked into grana (granum)
- stroma
12. Photosynthesis summary equation:
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O
Photosynthesis is a redox process:
- H2O is oxidized
- CO2 is reduced
13. Chloroplasts split H2O into hydrogen and oxygen,
incorporating the e- of H into glucose
Reactants: 6 CO2
Products:
12 H2O
6 O26 H2OC6H12O6
14. The Two Stages of Photosynthesis: A Preview
Light reactions: the “photo” part
Calvin cycle: the “synthesis” part
Light Reactions (in thylakoids):
- split H2O
- release O2
- reduce NADP+ to NADPH
- generate ATP from ADP by photophosphorylation
20. The Nature of Sunlight
Light is a form of electromagnetic energy (radiation)
- travels in rhythmic waves
Wavelength: distance between crests of waves
- determines the type of electromagnetic energy
21. Electromagnetic spectrum: the entire range of
electromagnetic radiation
Visible light: the wavelengths that produce colors
we can see
- includes wavelengths that drive PSN
- behaves as though it consists of discrete energy
“particles” (photons)
23. Photosynthetic Pigments: The Light Receptors
Pigments: chemicals that absorb visible light
- different pigments absorb different wavelengths
Wavelengths that are not absorbed are reflected or
transmitted
- leaves appear green because chlorophyll reflects and
transmits green light
25. Spectrophotometer: measures a pigment’s ability to
absorb different wavelengths
- sends light through pigments
- measures the fraction of light transmitted at each
wavelength
26. Galvanometer
Slit moves to
pass light
of selected
wavelength
White
light
Green
light
Blue
light
The low transmittance
(high absorption) reading
-chlorophyll absorbs most
blue light
High transmittance
(low absorption) reading
-chlorophyll absorbs very little
green light
Refracting
prism Photoelectric
tube
Chlorophyll
solution
TECHNIQUE
1
2 3
4
27. Absorption spectrum: graph plotting a pigment’s light
absorption vs. wavelength
- the absorption spectrum of chlorophyll a suggests
that violet-blue and red light work best for PSN
Action spectrum: profiles the relative effectiveness of
different wavelengths of radiation in driving PSN
28. Wavelength of light (nm)
(b) Action spectrum
(a) Absorption spectra
(c) Engelmann’s
experiment
Aerobic bacteria
RESULTS
Filament
of alga
Chloro-
phyll a Chlorophyll b
Carotenoids
500400 600 700
700600500400
29. Chlorophyll a: the main photosynthetic pigment
Accessory pigments:
- Chlorophyll b: broadens the PSN spectrum
- Carotenoids: absorb excess light that would
damage chlorophyll
30. Porphyrin ring:
light-absorbing “head” of molecule
-Mg atom at center
in chlorophyll aCH3
Hydrocarbon tail:
interacts with hydrophobic regions of
proteins inside thylakoid membranes of
chloroplasts
(H atoms not shown)
CHO in chlorophyll b
31. Excitation of Chlorophyll by Light
When a pigment absorbs light, it goes from a stable
ground state to an unstable excited state
When excited e- fall back to the ground state,
photons are given off (fluorescence)
- if illuminated, a chlorophyll solution will fluoresce,
giving off light and heat
32. (a) Excitation of isolated chlorophyll molecule
Heat
Excited
state
(b) Fluorescence
Photon
Ground
state
Photon
(fluorescence)
Energyofelectron
e–
Chlorophyll
molecule
e–
33. Photosystem consists of:
- Reaction-center complex surrounded by
- Light-harvesting complexes (pigments bound to
proteins)
Funnels energy of photons to the reaction center
34. Primary electron acceptor (in reaction center
complex)
- accepts an excited e- from chlorophyll a
- the 1st step of the light reactions
36. Two Types of Photosystems:
Photosystem II (PS II) functions first:
- best at absorbing wavelengths of 680 nm
P680: the reaction-center chlorophyll a of PS II
37. Photosystem I (PS I) functions second:
- best at absorbing wavelengths of 700 nm
P700: the reaction-center chlorophyll a of PS I
38. Linear Electron Flow
Two possible routes for e- flow during the light
reactions: cyclic and linear
Linear electron flow: the primary pathway
- involves both photosystems I and II
- produces ATP and NADPH using light energy
39. A photon hits a pigment and its energy is passed
among pigment molecules until it excites P680
- an excited e- from P680 is transferred to the
primary electron acceptor
41. P680+ = P680 with one less e-
- is a very strong oxidizing agent
- H2O is split by enzymes
- its e- are transferred from H atoms to P680+
- reduces P680+ to P680
- O2 is released as a by-product of this reaction
43. Each e- “falls” down an electron transport chain from
the primary electron acceptor of PS II to PS I
- energy released by the fall drives the creation of a
proton gradient across the thylakoid membrane
- the diffusion of H+ (protons) across the membrane
drives ATP synthesis
45. In PS I (like PS II), transferred light energy excites
P700, which loses an e- to an electron acceptor
P700+: P700 that is missing an e-
- accepts an e- passed down from PS II via the electron
transport chain
47. Each e- “falls” down an electron transport chain from
the primary electron acceptor of PS I to the protein
ferredoxin (Fd)
- the e- are then transferred to NADP+ and reduce it to
NADPH
- the e- of NADPH are available for the reactions of the
Calvin cycle
50. A Comparison of Chemiosmosis in Chloroplasts
and Mitochondria
Both generate ATP by chemiosmosis, but use
different sources of energy:
Mitochondria transfer chemical energy from food
to ATP
Chloroplasts transform light energy into the
chemical energy of ATP
51. Spatial Organization of Chemiosmosis:
Mitochondria: protons are pumped into the
intermembrane space
- diffusion of protons back into the matrix drives
ATP synthesis
Chloroplasts: protons are pumped into the
thylakoid space
- diffusion of protons back into the stroma drives
ATP synthesis
53. ATP and NADPH are produced on the side facing the
stroma, where the Calvin cycle takes place
In summary:
- light reactions generate ATP
- they increase the potential energy of e- by moving
them from H2O to NADPH
55. The Calvin cycle regenerates its starting material as
molecules enter and leave the cycle
- analagous to the citric acid cycle
- builds sugar from smaller molecules using ATP and
the reducing power of e- carried by NADPH
56. Carbon enters the Calvin cycle as CO2
Carbon leaves as the sugar, glyceraldehyde-3-
phospate (G3P)
For net synthesis of 1 G3P:
- the Calvin cycle must turn 3X, fixing 3 molecules of
CO2
57. Three Phases of the Calvin Cycle
1. Carbon fixation (catalyzed by rubisco)
2. Reduction
3. Regeneration of the CO2 acceptor (RuBP)
59. Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate
Short-lived
intermediate
Phase 1: Carbon fixation
(Entering one
at a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
ATP6
6 ADP
P P6
1,3-Bisphosphoglycerate
6
P
P6
6
6 NADP+
NADPH
i
Phase 2:
Reduction
Glyceraldehyde-3-phosphate
(G3P)
1 P
Output G3P
(a sugar)
Glucose and
other organic
compounds
Calvin
Cycle
60. Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate
Short-lived
intermediate
Phase 1: Carbon fixation
(Entering one
at a time)
Rubisco
Input
CO2
P
3 6
3
3
P
PPP
ATP6
6 ADP
P P6
1,3-Bisphosphoglycerate
6
P
P6
6
6 NADP+
NADPH
i
Phase 2:
Reduction
Glyceraldehyde-3-phosphate
(G3P)
1 P
Output G3P
(a sugar)
Glucose and
other organic
compounds
Calvin
Cycle
3
3 ADP
ATP
5 P
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
G3P
61. Light
Reactions:
Photosystem II
Electron transport chain
Photosystem I
Electron transport chain
CO2
NADP+
ADP
P i
+
RuBP 3-Phosphoglycerate
Calvin
Cycle
G3PATP
NADPH
Starch
(storage)
Sucrose (export)
Chloroplast
Light
H2O
O2